1. Field of the Invention:
This invention relates to a ferroresonant three-phase constant AC voltage transformer and, more particularly, to a ferroresonant three-phase constant AC voltage transformer capable of reducing a deviation that can be generated in the phase difference between the output voltages when unbalanced loads or unbalanced three-phase input power source voltages or both are connected thereto.
2. Description of the Prior Art:
A ferroresonant constant AC voltage circuit has a configuration wherein a series circuit consisting of a reactor L2 and a switching element SW is connected in parallel to an output capacitor C and to a load R with each of the latter two also being connected in parallel to each other. These parallel circuits and a reactor L1 are connected in series to an input source of voltage Ei as illustrated in FIG. 6. By controlling the ON-OFF time of the switching element SW with a negative feedback circuit FBC and consequently controlling the input current flowing through the reactor L1, the amount of the voltage drop between the opposite terminals of the reactor L1, which is serially connected between the input and output, can be regulated and the AC voltage Eo applied to the output or load can be kept constant (as disclosed in the specification of U.S. Pat. No. 4,642,549).
In the present specification, the output capacitor C, the reactor L2, the switching element SW, and the negative feedback circuit FBC may be referred to collectively as the "automatic voltage regulating" (AVR) part.
It is permissible, as is widely known, to utilize as the series reactor L1 a leakage inductance of a transformer T which is provided with a magnetic shunt Ms as illustrated in FIG. 7. In this arrangement, it is no longer necessary to add a series reactor as an external circuit component. FIG. 6, therefore, is an equivalent circuit of FIG. 7.
As examples of transformers provided with a magnetic shunt, not only diport transformers configured as illustrated in FIG. 7 but also triport transformers (Japanese patent application disclosure SHO 60(1985)-219,928 and Japanese patent application disclosure SHO 61(1986)-54,513) have been known to the art.
In the conventional constant voltage circuit described above, a phase difference occurs between the phase of the input voltage Ei and the phase of the output voltage Eo because the output voltage Eo is regulated to a target (fixed) value by controlling the magnitude of the electric current flowing in the reactor L1 which is serially connected between the input and output. This phase difference depends on the magnitude of the output current and the power factor of the output (load R). When three constant voltage circuits such as described above are assembled in a three-phase connection and utilized as a three-phase power source, deviations in the phase differences between the input and output voltages cause deviations between the phases of the three output phase voltages.
When the output load is balanced among the three phases, the deviations in phase between the input and output voltages are equal for all three phases. Each of the phase differences between the output voltage phases is 120 degrees for each of the phase differences between the three input voltage phases being 120 degrees. When the load is unbalanced, the phase difference between the input and output voltages is likewise unbalanced among the phases and, as a result, the phase differences of the output phase voltages deviate from 120 degrees.
For example, in a three-phase constant voltage circuit using three diport transformers T1 to T3 as illustrated in FIG. 8, the voltage vectors which are obtained when a load R is applied only on the output U phase of the circuit and no load is applied to the other V and W phases will be as illustrated in FIG. 9.
In the circuit of FIG. 8 there is connected in series, with each of primary (input) windings 12, 22, and 32 of diport transformers T1 to T3 a corresponding one of series reactors L1r to L1t, respectively. These three series reactor-primary winding sets are joined together as phase windings in a delta-connection having input terminals R, S, and T.
The secondary (output) terminals of the diport transformers have corresponding automatic voltage regulating means AVRu to AVRw, of the same configurations as in FIG. 6 and FIG. 7, joined together in a Y connection. N stands for a neutral point. In this case, as clearly noted from the diagrams, a voltage drop V1 occurs only in the series reactor L1r of the U phase while no voltage drop occurs in the reactors L1s and L1t of the V phase and the W phase, respectively. As a result, a phase delay of an amount .phi. occurs as illustrated in FIG. 9 in the voltage vector Vun of the output voltage on output U while no phase delay occurs in the voltage vectors Vvn and Vwn of the other voltages present on outputs V and W.
As a result, there arises a loss of balance in the output voltages. The resulting phase differences between the output voltages becomes (120-.phi.) degrees between voltages on outputs U and V, 120 degrees between those on outputs V and W, and (120+.phi.) degrees between those on outputs W and U.
When such a deviation occurs in the phases of the output voltages of a three-phase power source device, a three-phase motor used as a load may show a decrease in driving torque and may generate a torque ripple to provide a possible cause for noise. When a frequency tripler (multiplier) is used, the deviation of the sort mentioned above may impair the frequency multiplier's capacity for operation. In an extreme case, this deviation may prevent the frequency multiplier from effecting the multiplication aimed at, may degrade the frequency multiplier's capability for keeping a constant output voltage, and may entail various other similar drawbacks.
In the United States, for example, the deviation in the phase difference is required to be prevented from exceeding 3 degrees in a 30% unbalanced load (a load operated under the conditions of 70% in the U phase, 100% in the V phase, and 100% in the W phase, for example). Any attempt at meeting this requirement, however, entails a degradation of the power factor. It is not easy to keep both phase difference and power factor within their allowable limits.
One conceivable way of diminishing the deviation in the phase difference may consist of decreasing the series reactance. This measure, however, entails a disadvantage in that the power capacity on the primary side must be increased because the constant voltage characteristic is degraded and the current-limiting effect to be manifested in the case of secondary short circuit is impaired.
This invention has been made for the purpose of substantially overcoming the drawbacks of the prior art mentioned above.